The whole MCU is basically running off the B and C list of characters.
Y'know, the ones no-one actually cared enough to license back when Marvel was trying to get cash by selling off the family jewels like the X-Men, Hulk, Fantastic Four and especially Spider-Man.
So they sold off all the good villains as well. Leaving them with also ran shit like Iron Man's villains, the main one of whom is essentially a massively racist generic yellow peril Fu-Manchu ripoff. And other than him no-one gives a shit about any of Iron Man's villains, which is why they're such small parts of his own movies whilst he wrestles with his own self destructive tendencies.
Likewise Thor. Thor is a fucking nobody, name two Thor villains that aren't Loki? And I'll even give you Enchantress for free.
Captain America hardly even has villains to call his own, in the movies he has to borrow Hydra, originally Nick Fury's nemesis! (Not that they tried hard with Red Skull, he's so evil he's a double Nazi, 100% characterisation achieved!)
That's why the MCU is running on also ran villains, because it's running on heroes that no-one gives a fuck about in the first place. The really iconic Marvel antagonists are all either X-Men, Fantastic Four*, or Spider-Man.
* PS: It always turns out to be Doom in the end.